Most primates are active either in the day or night, but camera traps are revealing that some monkey and chimp day dwellers also go out at night for things such as pool soaks and snacks.

The latest to be snapped unaware is the Guizhou snub-nosed monkey, Rhinopithecus brelichi. Once thought to be exclusively diurnal, this Asian monkey's nightlife is documented in the latest issue of the journal Primates.

"Our camera trap photos showed Guizhou snub-nosed monkeys moving in trees at night," lead author Chia Tan told Discovery News. "We believe the monkeys were on their way to search for food."

Tan, who works at the San Diego Zoo Institute for Conservation Research, and her colleagues Yeqin Yang and Kefeng Niu spied the monkeys after setting out the camera traps. Guizhou snub-nosed monkeys are endangered, with just a single global population of 700-800 individuals restricted to Fanjingshan in southwest China.

The monkeys were active during both day and night all year round, but they went out more at night during the early spring and autumn.

"We think the monkeys are extending their activity beyond daylight hours to increase feeding, and the highly sought-after food items are young leaves in spring and fruit and seeds in autumn," Tan explained.

She added, "It makes sense that the monkeys take advantage of these super nutritious foods to maximize their reproduction and survival. Spring and autumn are critical times for the monkeys; they are the birthing and mating seasons, respectively."

Since their forest home is often foggy, the researchers suspect that the monkeys may have evolved the ability to see under low light conditions. Poor eyesight along with night predators, such as the clouded leopard, would make for a potentially disastrous combination, but the monkeys seem to have mostly overcome it.

Chia Tan

Guizhou snub-nosed monkeys are an endangered species.

Other primates that are active during the day have been found to have nightlives too. The well-named owl monkey, for example, is known for its nocturnal ways.

Humans who have swum on a hot summer's night will also appreciate what was discovered about our closest living primate relatives.

Eduardo Fernandez-Duque, an associate professor of anthropology at the University of Pennsylvania, told Discovery News that Tan's team used a "novel approach to the study of primate activity."

He added, "The article combines the use of some new technology with traditional approaches to learn that the snub-nosed monkeys, traditionally considered diurnal, may show some nocturnal activity under certain circumstances. Their proposition that this flexibility in activity patterns may be associated to the temperate environment is reasonable."

Humans, of course, fall into the day and/or night-living primate category too. This is often a survival tactic, as people with late shift jobs could attest. It's possible that our distant relatives were 24-7 individuals as well.

"It is difficult to infer a full range of behavior from fossilized species, but I would not be surprised if some of our earliest human ancestors were regular night owls," Tan said. "They would, under certain environmental conditions, be up at night. Party anyone?"